1. Field of the Invention
The present invention relates to techniques for computing apparent motion in sequences of images. More specifically, the present invention relates to a method and an apparatus for using directional weighting functions during a belief propagation process which computes an optical flow field for an image.
2. Related Art
In order to enable computer systems to process visual information, computer scientists have developed a number of techniques to represent visual motion. Many of these techniques involve computing “optical flow” for an image in a video sequence. An optical flow field is a two-dimensional velocity field describing apparent motion within an image, which results from the independent motion of objects in the image, or from motion of the observer, projected onto the image plane. The process of computing optical flow for an image in a video sequence involves analyzing differences between two or more successive images in the video sequence to determine the apparent motion of objects between the images.
Optical flow can be computed using a number of techniques. Some researchers have investigated techniques that use “belief propagation” to calculate optical flow. (see P. F. Felzenszwalb and D. P. Huttenlocher, “Efficient Belief Propagation for Early Vision,” CVPR, vol. I, pp. 261-268, 2004.) Belief propagation has proven to be an effective technique for solving a large number of computer vision problems. It operates by associating pixels in an image with nodes in a belief propagation graph. During the belief propagation process, the system performs local computations for nodes in the belief propagation graph and iteratively communicates these “messages” to neighboring nodes.
Unfortunately, existing techniques for solving the optical flow problem (including the above-described belief propagation technique) have a number of shortcomings. One shortcoming is caused by the “aperture problem,” which arises whenever a small neighborhood in an image does not sufficiently constrain the motion of that neighborhood. The aperture problem commonly occurs along edges of an object, where only the motion normal to the edge is known, and in textureless regions, where motion is unconstrained.
Moreover, the optical flow problem is exacerbated if information is passed across motion (object) boundaries, where discontinuities exist in the optical flow. Note that velocity information on one side of an object boundary is typically irrelevant for points that lie on the other side of the object boundary. For example, the velocities of points within an object are typically irrelevant in determining the velocity of points that lie outside of the object. Consequently, transferring this velocity information across object boundaries during the belief propagation process can cause optical flow computations to converge to erroneous results.
Hence, what is needed is a method and an apparatus that restricts information from being passed across motion boundaries during optical flow computations involving the belief propagation technique.